In general, several different types of sensing apparatus have been used to measure current in an electrical circuit. Such sensing apparatus may utilize sense resistors, current transformers, and Hall Effect sensors. These sensors are used to monitor and measure current as it is passes through the load of a circuit either for purposes of overcurrent protection or to control another circuit based on the measured current in the load.
Sensors are also used to detect wave forms and the shape of current which can indicate the presence of an arc fault. Arcing faults may be defined as the existence of a current path between two ends of a broken conductor located within an ionized gas, between two conductors supplying a load, or between a conductor and a ground. Arcing faults are characterized by low and erratic current flow. Arcing faults may be undetected by standard circuit breakers, because the current flow may be below the breaker's tripping threshold. Upon occurrence of an arcing fault, branch or load impedance may cause the current levels to be reduced to a level below the trip curve setting of the circuit breaker, causing the arcing fault condition to be undetected by a circuit breaker. In addition, an arcing fault which does not contact a grounded conductor or other grounded point will not trip a ground fault protected circuit.
During the current measurement, it is important to maintain galvanic isolation in order to assure that current does not flow directly between the load and the measuring circuit. Sense resistors, typically, are accurate, inexpensive, and provide a higher frequency response than the other two types of sensors mentioned above. However, a disadvantage of the sense resistors is that they provide no galvanic isolation. Another disadvantage is that the sensor output signal tends to vary with temperature variations. This is due to the composition of the sensing material used. Metals have an inherent property of changing resistance when ambient temperatures fluctuate. By way of example, the proportionality of a signal being converted at 25 degrees Celsius will be different from the same signal being converted at 66 degrees Celsius or −35 degrees Celsius. Similarly, the sensitivity of the circuit changes if there is a deviation from a nominal operational temperature of 25 degrees Celsius. Known sense resistor configurations do not compensate the current for this variance in ambient temperature. Therefore, a need exists to compensate for temperature induced signal drift.
On the other hand, both current transformers and Hall Effect sensors provide galvanic isolation. Current transformers also provide proportional signals with suitable accuracy at a wide operational temperature range. However, both current transformers and Hall Effect sensors are more expensive solutions. Further, the frequency content for current transformer and Hall Effect sensors will be lost or will be filtered out because they have a low pass filter type transfer function. A disadvantage of a low pass filter type transfer function is that the critical information of the frequency content provided at higher frequencies will be missed. A further disadvantage of current transformers is that they occupy a relatively large area of a printed circuit board.